Lesson · Problem 19
Find Why It Fails
Evidence before a fix
You should be able to
- Read an authored panel as evidence, not as a live measurement of your edit.
- Change one suspected cause at a time, then recheck connectivity.
Debugging · Power · Mixed Signal
What it is
The panels are authored measurements of one faulty prototype. They do not update when you edit, and they are not a live oscilloscope. Read what each panel shows, decide what kind of fault could produce it, then change that part of the design. The lesson does not name the five repairs. The challenge is where you make that call.
Why this matters
A board that misbehaves has a cause. Changing three parts at random hides which change mattered.
What you are building
A faulty MCU, USB, and ADC board. You match authored panels to causes and then change the circuit.
Prerequisites
This lesson uses decoupling capacitor, bulk capacitor, reset, pull-up, D+, D−, ESD, series resistor, VREF, sampling, return path, aggressor. It introduces authored evidence, symptom, one change at a time.
Component guide
What the parts are. The requirements panel is still the list that is graded.
Authored evidence
- What it does
- Measurements written for the faulty prototype. They do not update when you edit.
- Why it is here
- You match a panel to a cause. The panel is not a live oscilloscope.
- Symbol
- The Reference panels beside the sheet.
- Beginner mistake
- Replacing the MCU because several panels failed at once.
- What an engineer checks
- Read what each panel says changed, and what stayed still.
Capacitor
- What it does
- Stores charge and supplies a short pulse of current. With a resistor it also sets a time.
- Why it is here
- A supply capacitor sits at a pin. A filter capacitor sits in the signal path. They are different jobs.
- Symbol
- Two parallel plates. A polarized part adds a plus on one plate.
- Footprint
- Two pads. Polarized parts mark the positive pad.
- Polarity
- A ceramic bypass in this course is not polarized. An electrolytic is. The card says which.
- Values
- 100 nF is the usual local bypass here. 1 µF and 10 µF show up where the card asks for bulk.
- Beginner mistake
- Using the filter capacitor as the supply capacitor, or one capacitor for three supply pins.
- What an engineer checks
- Capacitance, voltage rating, and whether it is polarized. The card states the value that is graded.
USB pair on this board
- What it does
- D+ and D− through the connector, the series resistors, and the ESD parts.
- Why it is here
- One of the panels is about that path. The lesson does not name which repair it is.
- Symbol
- The USB-C symbol and the pair.
- Beginner mistake
- Swapping the pair because a panel mentioned USB.
- What an engineer checks
- Pin names on the connector, after you have a cause for the panel.
Interview lens
Separate a symptom that follows the load from one that follows a clock. Ask what stayed still.
What PCBGrade measures
The repair against the published rules. The panels stay as written. They are not a live scope.
Where this shows up
A lab debug starts from what the instrument showed, not from replacing the most expensive part.
Terms
- Load step
- A sudden change in current. The supply voltage dips if the local capacitors cannot supply it.
- Authored evidence
- Measurements written for the scenario. They do not update when you edit.
What is happening electrically
A symptom is what the board does. A cause is the part or the geometry that makes it do that. A supply that dips when the load steps is a charge problem. A reset pin that chatters while the supply is steady is a pin that is not held. A link that attaches as the wrong speed is a data path that does not match the connector. Codes that grow with the input are a reference problem. Noise that follows the clock is coupling. Those are different physics.
Why the geometry matters
The copper and the schematic are both evidence, and so are the panels. The panels were written for the faulty prototype. They do not move when you edit. After you change one thing, connectivity and the design rules still have to pass. A repair that explains a panel and shorts two nets is not finished.
How an engineer reasons
Write one sentence per panel: what changes, and what stays still. Match that sentence to a block you already know from earlier lessons. Change one suspected cause. Check. If a panel’s story does not match the block you touched, that was the wrong block. Replacing the microcontroller because several symptoms showed up at once skips the reasoning.
Worked example
Open Reference first. For each panel, write one sentence about what changed and what stayed still. Then look at the block that owns that behavior. After the repair, connectivity and the design rules still have to pass.
Good and bad
One change that fits one panel, then a recheck, teaches you whether that change was the cause. Three changes at once can make the board better and leave you unable to say why. A part you never touched cannot explain a symptom the prototype already had.
Common mistake
Replacing the MCU because several symptoms showed up at once.
Where this rule stops
The lesson does not name the repairs. The panels are authored evidence, not a live oscilloscope and not a measurement of your edit. The exact way each cause is scored stays in the collapsed measurement note on the challenge. Guessing the rubric from this page is not the skill.
Before the challenge
The challenge shows symptoms and authored panels. You decide what to change. This lesson does not name the repairs. After the change, connectivity and the design rules still have to pass.
Ready for the challenge
You can write one sentence per panel before you move a part.
Question 1 of 2